1. Field of the Invention
The present invention relates to a method of controlling a servo motor and a method of controlling a robot system when an abnormal load due to a collision with an object is detected.
2. Discussion of the Background
When an arm constituting an industrial robot or an end effector gripped by such an arm collides with an obstacle, a servo motor that continuously drives each arm, during abnormal load detection in response to a given movement command, causes the servo motor to be locked and to produce a large torque. If this situation lasts for an extended period of time, damage to a mechanical arm section including the servo motor and a reduction gear may result. Also, mechanical damage to the obstacle may result.
There have been proposed, for example, five methods described in the following documents for detecting the occurrence of a collision and immediately suspending a movement command issued to the servo motor; Japanese Unexamined Patent Application Publication No. 6-131050, Japanese Unexamined Patent Application Publication No. 3-196313, Japanese Unexamined Patent Application Publication No. 6-245561, Japanese Unexamined Patent Application Publication No. 6-278081, and International Publication WO 2005/009692.
When encountering a collision with an obstacle, the robot system proposed in the above documents can reduce a collision torque by switching its operation mode to a flexible control mode so as to behave in accordance with the strength of a collision force if the rotational direction of a motor is the same as the direction of the collision torque. However, such a switch to the flexible control mode may cause the robot to go out of control, and the robot and the obstacle are subjected to a great impact force if the robot has large inertia at the time of the collision. In particular, if the servo motor driving the robot around the fundamental axis is running at a high speed at the time of the collision, the switch to the flexible control mode causes the robot to move a large distance until its inertia becomes zero, which may cause another collision, resulting in significant mechanical damage to the robot and the obstacle. Also, if all of the joints used in the robot are subjected to the flexible control, the robot largely deviates from a commanded trajectory. For this reason, when operated again after the detection of the collision, the robot moves through a trajectory other than that commanded at the time of teaching, which may cause a collision with an obstacle.